The invention relates to a fuel metering apparatus in an internal combustion engine. It is generally known to make the fuel metering signal, intended for injection valves, for instance, dependent on the exhaust gas composition. See U.S. Pat. No. 3,745,768 to Zechnall. To this end, the oxygen component of the exhaust gas is detected by a sensor and evaluated for the purpose of regulating the mixture composition. Oxygen sensors, however, are not failsafe, so monitoring devices must be provided. For example, the impulse of the output signal in the lambda sensor, or oxygen sensor, becomes flatter with increased operating time, which is detrimental to the reaction time of the apparatus controlling the mixture composition.
A monitoring device for an oxygen sensor is already known which has two threshold switches which, during the transition of the mixture (a transition from rich to lean, for example), detects the dwell time of the sensor output signal in a predetermined middle range and evaluates the signal to recognize an error. See U.S. Pat. No. 4,208,993 to Peter. Not only does this involve increased expense, but the use of two comparators has proved to be not suitable for safety reasons.